Motivated by the demand for achieving high modulation depth of optical signal in three-dimensional (3D) imaging light detection and ranging (LIDAR), the electro-optic (EO) modulation depth of KTa1−x NbxO3 (KTN) crystal and its stability under the electric field have been investigated. The strong field-enhanced effect of dielectric permittivity occurs owing to the dielectric resonance, the modulation depth is enhanced, and half-wave voltage is further reduced. Only when the alternating current (AC) electric field is applied, the modulation depth increases with the increment of the electric field and trends to remain unchanged under the high owing to the saturation of polar nanoregions (PNRs) in KTN crystal. The modulation depth fluctuates with time, which can reduce the ranging accuracy of 3D imaging LIDAR. We found it can be attributed to the instability of PNRs. After applying the direct current (DC) electric field, the modulation depth was increased and its stability was enhanced owing to the improved polarization and enhanced stability of PNRs. The result reveals that the DC + AC electric field can increase the ranging accuracy of LIDAR compared to the AC electric field. We will provide guidance for designing the EO modulator in high-accuracy 3D imaging LIDAR from the perspective of nanodomain engineering and electric field-driven systems.